4 INTRODUCTION 



the smallest possible surface to the stimulus of the current; 

 or, again, a piece of food will attract it, and towards that 

 piece of food it will slowly move — ^in short it responds to 

 external stimuli, and is, as the phj^siologists call it, irritable. 



These activities and qualities imply a certain expenditure 

 of energy; hoAV is that energy supplied? What is the oil that 

 drives this engine? It is the food already hinted at. Living 

 protoplasm must have food. It takes to itself certain food 

 substances of a high complexity and oxidizes and reduces 

 these to simpler substances, and during this process, just as 

 when gunpowder explodes, energy and heat are set free. It is 

 also capable of building up the dead food into its own flesh 

 (or into protoplasm), making the dead live, and this quality 

 is called assimilation. Further, all protoplasm breathes; that 

 is to say, it takes in oxygen and it gives out carbon dioxide. 

 It is in effect respiratory. Should the supply of oxygen in this 

 world of ours be suddenly withdrawn, all life would cease, 

 and in the course of a few weeks or months the whole fabric 

 of our earth would have become mineralized. Life would 

 cease. 



Living animals and plants secrete, that is to say, certain 

 cells and organs bring forth products which play a large part 

 in the life of the animal or plant in question. Examples of 

 such glands or secretory cells in plants are found in the 

 nectaries of flowers, which secrete a sugary fluid attractive 

 to insects, who bring ^\ith them pollen from other plants to 

 fertilize the flower. The glands of insectivorous plants pro- 

 duce a fluid which digests the proteins of the insects they 

 catch, and other cells secrete certain solvents which render 

 the starch in seeds soluble. 



In the higher animals the products of the secretory glands 

 play a large part in digestion. The salivary glands secrete 

 saliva into the mouth, which moistens the food and turns 

 starch into sugar. The w^alls of the stomach secrete gastric 

 juice, which together ^^dth the products of the liver and the 

 pancreas help to render undigested food soluble so that it 

 may be taken up into the blood-stream. Then there are certain 

 small glands which have no ducts and do not open anywhere. 

 But their secretion passes into the blood and plays a large 



